GB/T 38894-2020 English PDF (GBT38894-2020)

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GB/T 38894-2020: Non-destructive testing--Electrochemical testing--General principles
GB/T 38894-2020
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 19.100
J 04
Non-destructive testing - Electrochemical testing -
General principles
ISSUED ON: JUNE 02, 2020
IMPLEMENTED ON: DECEMBER 01, 2020
Issued by: State Administration for Market Regulation;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3 
1 Scope ... 4 
2 Normative references ... 4 
3 Terms and definitions ... 4 
4 Testing principle ... 5 
5 Personnel requirements ... 12 
6 Electrochemical testing equipment ... 12 
7 Testing ... 14 
8 Test report ... 15 
Non-destructive testing - Electrochemical testing -
General principles
1 Scope
This standard stipulates the general principles of non-destructive
electrochemical testing such as selective electrodes, electrochemical
impedance, multiple potential scanning for metal materials and products, as
well as steel in infrastructure.
This standard applies to non-destructive testing of the corrosion state, corrosion
tendency and corrosion rate of metal equipment and components in the service
environment.
2 Normative references
The following documents are essential to the application of this document. For
the dated documents, only the versions with the dates indicated are applicable
to this document; for the undated documents, only the latest version (including
all the amendments) are applicable to this standard.
GB/T 10123 Corrosion of metals and alloys - Basic terms and definitions
GB/T 33373 Anticorrosion - Electrochemical protection - Terminology
3 Terms and definitions
The terms and definitions as defined in GB/T 10123 and GB/T 33373 as well as
the following terms and definitions apply to this document.
3.1
Service environment
The environment in which metal equipment and its components are in use.
3.2
Multi circle potential dynamic polarization
At a certain potential scanning speed, apply repeated potential polarization
with an IrO2 film, where the metal titanium is a conductor and the IrO2 film is a
hydrogen ion sensitive film. In an aqueous environment, the following
electrochemical reactions occur on the surface of the Ti/IrO2 electrode:
2IrO2 + 2H+ + 2e → Ir2O3 + H2O
In the equilibrium state, the potential of the Ti/IrO2 electrode has a linear
relationship with the concentration of hydrogen ions in the solution. By
measuring the potential of the Ti/IrO2 electrode in a series of standard pH
solutions, the working curve of the pH sensor can be obtained. Measure the
potential of the pH sensor. According to the slope and intercept value of the
working curve, the pH value of the service environment can be converted.
4.1.4 Features
Selective electrode’s testing has the following features:
- Easy and fast operation, not affected by the color or turbidity of the
environmental medium;
- It may carry out tracking monitoring and automatic testing.
4.1.5 Limitations
Selective electrode testing has the following limitations:
- Other species in the environmental medium may interfere with the testing;
- The selective electrode has temperature deviation in the testing of species
in the environment, which needs to be compensated and corrected.
4.2 Corrosion potential testing
4.2.1 Overview
Corrosion potential is an important corrosion electrochemical parameter, which
can judge the corrosion tendency of metals; it is also an important parameter in
electrochemical protection.
4.2.2 Selection of reference electrode
The selection of the reference electrode shall meet the following requirements:
- High stability in the service environment;
- Allows a small current to pass without electrode polarization;
- It has good durability in the service environment;
4.3.1 Overview
Apply a small amplitude AC disturbance signal to the metal corrosion system;
measure the corresponding AC response signal; calculate the ratio of voltage
to current (that is, the impedance of the corrosion system) or the phase angle
change with the frequency of the sine wave; then analyze the dynamic process
of the metal corrosion system and corrosion rate.
4.3.2 Basic conditions for testing
Electrochemical impedance measurement shall meet the following conditions:
- Causality conditions: When a sine wave potential signal is used to perturb
the corrosion system, the corrosion system only responds to the potential
signal;
- Linearity conditions: There is an approximately linear relationship between
the response signal output by the corrosion system and the input
disturbance signal;
- Stability conditions: After stopping the disturbance to the corrosion system,
the corrosion system can return to its original state.
4.3.3 Electrochemical impedance spectroscopy
Use a small amplitude sine wave potential (or current) in a certain frequency
range as a disturbance signal, to excite the metal system in the service
environment. By measuring the corresponding alternating current (or potential)
response signal, the electrochemical impedance spectroscopy of the metal in
the service environment is obtained in situ. Based on the electrochemical
electrode process theory and the fitting of the equivalent circuit, explore the
mechanism of the corrosion reaction kinetic process, estimate the corrosion
electrode’s reaction speed and step-by-step kinetic parameters.
Electrochemical impedance measurement should use special instruments and
analysis software. For the testing of metal corrosion rate in a specific
environment, sometimes the characteristic frequency’s fixed frequency
measurement can be selected, to realize the electrochemical impedance
method to quickly detect the metal corrosion rate.
Electrochemical impedance spectroscopy usually uses an equivalent circuit to
analyze the value of its components, which reflects the interface structure of
the metal corrosion process and the kinetic parameters of each reaction step.
The equivalent circuit of the most common metal corrosion system is as shown
in Figure 1, where Rs is the resistance of the environmental medium; C is the
capacitance of the metal surface; Rp is the polarization resistance, which
reflects the resistance of the metal to the corrosion reaction. The value is
inversely proportional to the corrosion rate of metal.
Selective electrode testing equipment shall include selective ion potential
detection module, control module and communication module, etc.
Electrochemical impedance testing equipment shall include a constant potential
module, a signal generator, a current signal acquisition module, a control
module, a communication module, etc.
Multiple potential scanning testing equipment shall include a constant potential
module, a polarization current testing module, a control module, a
communication module.
6.2 Testing/monitoring system
According to the purpose of testing, select the appropriate testing method and
system; various technical parameters shall be stated in the application
documents, meanwhile meet the applicable standard requirements. When
choosing a testing system, other factors to consider include:
- The production process conditions of the equipment under test;
- Whether there is cathodic protection and stray current influence around the
component or structure under test;
- The temperature and humidity of the service environment;
- Chemical, electrical or mechanical interference conditions in the service
environment;
- According to ...